OBJECTIVE To determine if increased cell turnover in chronic myeloproliferative disorders can lead to hyperhomocysteinemia as a result of folate and/or cobalamin depletion, and contribute to oxidative stress. METHODS The clinical role of oxidative stress was investigated by measuring reactive oxygen species (ROS), total antioxidant capacity (TAC), and total homocysteine (tHcy), folate, cobalamin, and holotranscobalamin (HoloTC) levels in 51 chronic myeloproliferative disorders patients (male-to-female ratio: 1.1; median age: 64 years; range, 40-84 years), including 42 with primary myelofibrosis and 9 with post-polycythemia vera myelofibrosis. RESULTS Myelofibrotic patients had higher tHcy (p = 0.0201) and an unbalanced oxidative status (higher ROS and lower TAC levels; p < 0.0001) than controls. Presence of diabetes or another neoplasia was associated with higher ROS levels (p < 0.05), splenomegaly, hepatomegaly, and peripheral blasts with lower HoloTC levels (p < 0.005). The most severe forms of myelofibrosis (2-3) were associated with lower TAC (p = 0.045) and HoloTC levels (p = 0.017). Patients with Janus kinase-2 mutations had lower HoloTC levels (p = 0.0059). HoloTC deficiency was more frequently associated with Janus kinase-2 homozygosity (p < 0.0003). CONCLUSIONS Our findings suggest that the determination of HoloTC, tHcy, ROS concentrations, and TAC, can identify latent cobalamin deficiency and provide a rational basis for correcting the increased oxidation associated with disease progression.